1. Field of the Invention
The present invention relates in general to superconducting material, and in particular to a new and improved method and apparatus of producing elongated flexible fibers from such material.
2. Description of the Related Art
U.S. Pat. Nos. 4,299,861 and 4,078,747 produce flexible superconductor fibers by providing a superconducting layer on a carbon fiber. U.S. Pat. No. 4,861,751 is similar in that the superconductor is formed as a sheath of superconducting oxide exterior to a core of amorphous metal alloy. U.S. Pat. No. 3,951,870 also relates to preparing a flexible superconductor fiber by the chemical conversion of a precursor carbon fiber by the high temperature reaction of a carbon yarn with a transition metal such as NbCl.sub.5, H.sub.2, N.sub.2. U.S. Pat. No. 4,378,330 discloses a process for preparing a composite superconducting wire to form a plurality of very fine ductile superconductors in a ductile copper matrix. U.S. Pat. No. 4,939,308 discloses an electrodeposition method for forming a superconducting ceramic. U.S. Pat. No. 4,866,031 discloses a process for making 90.degree. K superconductors from acetate precursor solutions.
None of these references, however, addresses the problem of fiber brittleness where the fiber is of superconducting material only.
U.S. Pat. No. 4,828,469, which is owned by the assignee of the present application, discloses an improved nozzle for the production of alumina-silica ceramic fibers. The fibers from superconducting material produced with this nozzle are extremely brittle.
Also, see the article entitled "Preparation of Superconducting Bi--Sr--Ca--Cu--O Fibers" by LeBeau, et al., Appl. Phys. Lett., 55 (3) Jul. 17, 1989, which discloses long slender fibers of superconducting Bi compounds but which lacks the specific disclosure of the present application for creating these fibers.
Major advances have been made in the development of high-temperature superconductor (HTSC) materials based on copper-bearing oxides such as Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 and Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.x. These and other raw materials have been processed using a wide variety of techniques in an attempt to produce useful engineering devices. Some of the processing techniques used include plasma spraying, sputtering, sol-gel, laser pedestal growth, wire and strip manufacturing and fiberization. In the plasma spraying and sputtering methods, the HTSC material is deposited on a substrate to produce a thin film. In the laser-heated pedestal growth method, the HTSC powder is pressed into pellets and sintered and small rods are cut from the pellets. A laser is used to melt the top of the rod and a seed crystal is placed in the melt. The wire is grown by withdrawing the seed at a controlled rate between 1.5 and 50 mm/hr. This method is extremely slow and therefore does not lend itself to becoming a good technique for mass production.
In the fiberization method, Bismuth based compounds were melted and fiberized using a gas jet. Fibers typically 100 .mu.m to 200 .mu.m in diameter and 5 mm to 10 mm in length were produced using the nozzle from U.S. Pat. No. 4,828,469. The fibers were very brittle and did not have a large length-to-diameter ratio, however. Small pieces of thin film, strip, tape and wire have been produced from the superconducting materials.
With the development of gas fiberization techniques by The Babcock & Wilcox Company, the preparation of the high temperature superconductor Bi.sub.2 Sr.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8 from the melt became possible. The advantage of such an approach over commonly utilized powder sintering processes is that the material produced is (for practical purposes) amorphous with excellent ductility. Transformation of the amorphous product by crystallization via thermal treatment can be achieved reliably. Also, much higher densities than can be achieved from conventional processing are observed for the fiber material. A higher integrity structure with better current transport properties can be manufactured from such a starting stock material.
It is desirable with such a gas fiberization technique to complete the transformation of the molten droplet to a completely full length fiber by allowing the feeder ball droplet to stay molten until complete fiber transformation is completed.